Computed Radiography (CR) systems using stimulable phosphor sheets are well known clinical imaging tools. In a CR system, radiation is passed through a subject and impinges upon a stimulable phosphor sheet, commonly referred to as a CR plate, phosphor plate, or CR sheet, that stores a portion of the radiation energy as a latent image. After exposure to the radiation, the stimulable phosphor on the CR plate is subsequently scanned using an excitation light, such as a visible light or laser beam, in order to emit the stored image.
Some CR scanning systems employ a flying-spot scanning mechanism, in which a single laser beam is scanned across the CR plate in a raster pattern. The resulting excitation that provides the stored image is then directed to a sensor, providing a single point of image data at a time. Other CR systems provide a full line of image data at a time, offering advantages of faster throughput and lower cost and complexity over flying-spot scanners. For example, U.S. Pat. No. 6,373,074 (Mueller et al.) entitled “Device for Reading Out Information Stored in a Phosphor-Carrier, and an X-Ray Cassette” is directed to a CR system that scans a full line of image data points at a time.
FIG. 1 shows the basic components of a prior art CR optical scanning system 10. A linear light source 12, typically using an array of laser diodes or other light sources, directs a linear scanning beam 14 onto a stimulable phosphor sheet 16 that has been irradiated and stores a latent X-ray image. One or more cylindrical lenses 18 are used to direct the highly asymmetric linear output beam along a line 20 on the surface of phosphor sheet 16. In a sensing head 22, collection optics 24 then direct the stimulated light from line 20 on phosphor sheet 16 through an optical filter 26 and to a linear photodetector array 28, typically a CCD (charge-coupled device) array. Phosphor sheet 16 is indexed in direction D by a transport mechanism 60 to provide a scanning motion. In this way, phosphor sheet 16 is moved past sensing head 22 to detect each line of the image stored thereon. The sensed image data is then processed by an image processor 30 that assembles a two-dimensional output image from each successive sensed line. The output image can then be stored or displayed.
There have been a number of features proposed for improving the performance of CR plate scanner optics. Several examples are noted below.
U.S. Patent Application Publication No. 2003/0010945 entitled “Radiation Image Read-Out Apparatus” (Ishikawa) is directed to a light projection apparatus for projecting a line of stimulating light from an array of laser diodes.
U.S. Patent Application Publication No. 2002/0096653 entitled “Radiation Image Information Read-Out Apparatus” (Karasawa) relates to the use of condenser lens chromatic characteristics for isolating stimulated light from stimulating light provided from the array of laser diodes.
U.S. Patent Application Publication No. 2002/0056817 entitled “Radiation Image Information Reading Recording Apparatus” (Furue) is directed to a reading apparatus for obtaining the stored image from an irradiated stimulable phosphor sheet using an array of laser diodes.
U.S. Patent Application Publication No. 2002/0040972 entitled “Radiation Image Read-Out Method and Apparatus” (Arakawa) relates to an optical reading head using an array of laser diodes that employs a grid pattern for sensing each line of the stored image.
U.S. Patent Application Publication No. 2002/0100887 entitled “Radiation-Image Data Readout Apparatus and Line Sensor to be Utilized Therein” (Hagiwara et al.) relates to an improved sensing arrangement in a scanning head for a stimulable phosphor sheet.
U.S. Patent Application Publication No. 2001/0025936 entitled “Image Detecting Device and Readout Exposure Apparatus Therefore” (Shoji) is directed to an illumination apparatus using pairs of cylindrical lenses and a slit for conditioning light from an LED array or other linear array of light sources.
U.S. Patent Application Publication No. 2001/0028047 entitled “Radiation Image Read-Out Apparatus” (Isoda) relates to a system using conventional optical techniques with improvements to line sensor components for obtaining a larger percentage of the stimulated light.
U.S. Pat. No. 5,721,416 entitled “Optics for Forming a Sharp Illuminating Line of a Laser Beam” (Burghardt et al.) is directed to the use of a homogenizing optical system for conditioning a laser beam, such as a system that utilizes an arrangement of specially configured lens elements for spreading the incident laser beam over a broadened area, such as described in U.S. Pat. No. 5,414,559 (Burghardt et al.).
U.S. Patent Application Publication No. 2003/0128543 entitled “Apparatus for Projecting a Line of Light from a Diode-Laser Array” (Rekow) discloses an apparatus for forming a line of light from a diode laser bar, using an arrangement of anamorphic lenses, including cylindrical microlens arrays.
U.S. Pat. No. 6,565,248 entitled “Light Guide, Line Illumination Apparatus, and Image Acquisition System” (Honguh et al.) discloses a system using LED light sources and scattering marks arranged within a light guide, where the scattering marks are positioned near the focal point formed by an elliptical surface portion of the light guide, so that light is directed toward a surface to be scanned at a preferred angle.
U.S. Pat. No. 6,744,033 entitled “Bar-Shaped Light Guide, Line-Illuminating Device Incorporated with the Bar-Shaped Light Guide and Contact-Type Image Sensor Incorporated with the Line-Illuminating Device” (Ikeda) discloses an elliptically shaped illuminating light guide using scatterers for redirecting LED illumination, similar to that of the '248 Honguh et al. patent.
U.S. Pat. No. 4,598,738 entitled “Apparatus for Projecting a Laser Beam in a Linear Pattern” (Ozaki) relates to the use of a concave mirror for redirecting laser illumination that has been reflected from a convex reflector disposed in front of the mirror, forming a line of illumination thereby.
While there have been improvements to apparatus and methods for obtaining the stored image on a CR plate, there is still need for increased efficiency and overall image quality. One area of particular interest relates to providing a linear illumination source that is less expensive and more robust than that provided by conventional approaches.
Referring back to FIG. 1, light source 12 directs a narrow line of light onto stimulable phosphor sheet 16 as scanning beam 14. For obtaining high levels of image quality, scanning beam 14 must be of sufficient intensity and must be uniform over the length of the scan line. When using an array of multiple light sources, such as LEDs or laser diodes, illumination performance is compromised by premature aging or failure of one or more individual elements in the array.
Referring to FIG. 2A (a top view) and FIG. 2B (a side view), there is shown the arrangement of a prior art conventional light source and light conditioning components for providing an illuminating line of stimulating radiation suitable as a scan line for CR plate sensing applications. Light source 12 has an array of LEDs 32, or other suitable light-emitting sources, that direct light through an aperture 44 and to a set of cylindrical lenses L1, L2, and L3. Lenses L1, L2 and L3 form line 20 of illumination for CR plate stimulation. This basic arrangement, with variation in the number of LEDs 32 and in the number and arrangement of cylindrical lens elements, provides a line of light that is sufficiently uniform, thin, and sharp for CR plate scanning, using an economical LED array. The length of line 20 is scalable and can be increased by adding more individual LEDs 32 to the array.
There are drawbacks to the arrangement shown in FIGS. 2A and 2B. Failure of one or more individual LEDs 32 can result in a loss of uniformity. Referring to FIG. 3A, there are shown two faulted LEDs 31 in a portion of the LED array of light source 12. The graph of FIG. 3B shows relative irradiance over the corresponding portion of the length of line 20. As a best-case baseline, curve 62 shows relative irradiance when all LEDs 32 emit light. For comparison, the dotted line of curve 64 shows relative irradiance with two faulted LEDs 31. The resulting nonuniformity of illuminating line 20 may result in corresponding non-uniformities in the diagnostic X-ray image obtained from phosphor sheet 16 (FIG. 1).
Methods such as the use of a slit, as shown in the '25936 Shoji application cited above, can help to distribute light more uniformly and compensate somewhat for individual light source failures. However, even with conventional methods for faulted LED 31 compensation, image quality suffers as a result of component failure.
For suitable contrast, scanning beam 14 must have sufficiently narrow width and sharp definition, so that only the line of the stored image currently being sensed is stimulated. A number of patents address the problem of correcting diffusion of scanning beam 14. For example, U.S. Pat. No. 6,597,008 entitled “Method of Reading a Radiation Image Converting Panel” to Umemoto et al. and U.S. Pat. No. 6,255,660 entitled “Stimulable Phosphor Sheet Having Divided Phosphor Layer” to Isoda et al. both note that diffusion of the excitation light is undesirable and disclose phosphor sheet design solutions to counteract this unwanted effect.
Thus, there are challenges to obtaining an illumination apparatus for CR plate scanning and other scanning applications that provides suitable linear illumination having sufficiently narrow width and having uniform irradiance along its length, using an array of LEDs or other Lambertian light emitters. In particular, there is a need for an illumination apparatus that is robust to failure of individual LEDs 32 or other light sources.
The present invention is directed to providing a linear illumination apparatus and method intended to overcome at least one disadvantage noted above.